A short wave infrared ray (SWIR) image sensor may be used when it is impossible to use existing image sensors in conditions, for example, in bad weather, on cloudy days, or during night, and thus, the SWIR image sensor has been widely used in various fields such as geology, bio-chemistry, and meteorology. With regard to the SWIR image sensor, a photodiode having a semiconductor layer formed of indium gallium arsenide (InGaAs) is mainly used due to characteristics of wavelength. Fine current output from the photodiode is transmitted to an external read-out circuit through an amplifier configured as a metal-oxide-semiconductor (MOS) circuit on pixels of an image sensor.
After the photodiode having the semiconductor layer formed of InGaAs is formed on an indium phospide (InP) substrate, and the amplifier are formed on a substrate as the MOS circuit, each pixel of the SWIR image sensor may be manufactured by electrically coupling the two substrates. In this case, however, problems about costs of the InP substrate, coupling reliability, a limit to resolution, etc. may exist. Therefore, researches into a monolithic integration for simultaneously forming photodiodes and an amplifier have been conducted. However, with regard to a monolithic integration method, a temperature condition of a process for forming photodiodes and that of a process for forming an amplifier on a silicon (Si) substrate are different, and thus, the processes for forming the photodiodes and the amplifier are separately performed according to time. Properties of the MOS circuit of embodying the amplifier formed prior to the process for forming the photodiodes may be degraded due to a shift of threshold voltage of a transistor that may occur when the photodiodes are formed.